As a material of a next-generation power semiconductor device, a group III nitride, for example, a GaN (gallium nitride)-based semiconductor is expected. The GaN-based semiconductor device has a band gap wider than silicon (Si) and, as compared to the Si semiconductor device, can realize high breakdown voltage and low power loss.
In the GaN-based transistor, generally, a high electron mobility transistor (HEMT) structure using a two-dimensional electron gas (2D EG) as a carrier is applied. A general HEMT is a normally-on transistor that is conductive also in a case where a voltage is not applied to the gate. For this reason, there is a problem in that it is difficult to realize a normally-off transistor that is not conductive unless a voltage is applied to the gate.
In a power supply circuit or the like handling high power of several hundreds of volts to one thousand volts, a normally-off operation is required from the safety point of view. Thus, a circuit configuration realizing a normally-off operation by applying a cascode connection of a GaN-based transistor of the normally-on type and a Si transistor of the normally-off type has been proposed.
In an inverter circuit of a control system to which a motor that is an inductive load is connected, when a switching device is turned off, a reflux current flows through the switching device from the motor. In a case where the circuit configuration described above is applied to a switching device of an inverter circuit of a motor control system, a reflux current flows through a body diode of a Si transistor of the normally-off type. Since the recovery characteristic of the body diode of the Si transistor is inferior, there is concern that a loss in the inverter circuit increases due to the circuit configuration described above when the reflux current flows.